Low-rank coals (coals of low degree of coalification) such as brown coal and sub-bituminous coal contain moisture in a large amount and hence are low in calorific value per unit mass and low in transportation efficiency. However, since low-rank coals are abundant in its reserves, they are used as a fuel after having been dried and then compression-molded into given sizes to enhance the calorific value per unit mass and handleability, from the standpoint of effective utilization of resources.
Low-rank coals come to have spontaneous-ignition properties when dried in order to heighten the efficiency of transportation. Therefore, a drying method capable of diminishing the spontaneous-ignition properties is required. In addition, since the drying of low-rank coals requires a large quantity of energy, an efficient and economical drying method is desired.
Proposed as a method for the drying is, for example, a method in which high-temperature dry coal obtained by contact with a high-temperature gas is sprayed with water in an amount suitable for removing the heat thereof (see JP-A-59-227979). However, although the spontaneous-ignition properties are reduced to some degree by the cooling of the dehydrated coal, there are cases where it still has spontaneous-ignition properties. Because of this, an oxidation step in which the spontaneous-ignition properties of the dehydrated coal are controlled is further required, resulting in poor production efficiency.
Meanwhile, as a drying method with high production efficiency, a drying method has been proposed in which, for example, a hydration is performed and then an oxidization with air is performed, thereby shortening the time period required for the treatment for inhibiting spontaneous ignition, such as the preparation of an oxidizing gas (see JP-A-2011-37938). In this technique, however, since dehydrated coal is introduced into water in the hydration, there are cases where the surface of the coal which has undergone the hydration has enhanced activity and the spontaneous-ignition properties cannot be sufficiently reduced even when the activity is reduced by the subsequent oxidation.